pedal Balance knob mod?

I have noticed lately. many effects have a couple modes.. with a switch. and it either this or that.
the new bass pedal from Way Huge is like that.. the pedal I wanna do the mod to is the Guyatone FZ-2000 pedal(Fuzz Face variant).. to get the best of both modes so what does the mod mean. but modes are active and the knob replaces the switch- thus is a balance between the two effect. any idea how hard it is to do?